The present invention relates to light emitting diodes and in particular relates to increasing the external quantum efficiency of light emitting diodes in which a growth substrate has been partially or entirely removed, and a carrier substrate has been added.
Light emitting diodes (LEDs) are a class of photonic semiconductor devices that convert an applied voltage into light by encouraging electron-hole recombination events in an appropriate semiconductor material. In turn, some or all of the energy released in the recombination event produces a photon.
Light emitting diodes share a number of the favorable characteristics of other semiconductor devices. These include generally robust physical characteristics, long lifetime, high reliability, and, depending upon the particular materials, low cost.
A number of terms are used herein that are common and well-understood in the industry. In such industry use, however, these terms are sometimes informally blended in their meaning. Accordingly, these terms will be used as precisely as possible herein, but in every case their meaning will be clear in context.
Accordingly, the term “diode” or “chip” typically refers to the structure that minimally includes two semiconductor portions of opposite conductivity types (p and n) along with some form of ohmic contacts to permit current to flow across the resulting p-n junction when a potential difference is applied.
The term “lamp” is used to designate a light emitting diode that is matched with an appropriate mechanical support and electrical contact and potentially a lens to form a discrete device that can be added to or included in electrical circuits or lighting fixtures or both.
As used herein, the term “package” typically refers to the placement of the semiconductor chip on an appropriate physical and electrical structure (sometimes as simple as a small piece of metal through which the electrical current is applied) along with a plastic lens (resin, epoxy, encapsulant) that provides some physical protection to the diode and can optically direct the light output.
Appropriate references about the structure and operation of light emitting diodes and diode lamps include Sze, PHYSICS OF SEMICONDUCTOR DEVICES, 2d Edition (1981) and Schubert, LIGHT-EMITTING DIODES, Cambridge University Press (2003).
The color emitted by an LED is largely defined by the material from which it is formed. Diodes formed of gallium arsenide (GaAs) and gallium phosphide (GaP) tend to emit photons in the lower energy (yellow, red, infrared) portions of the spectrum. Materials such as silicon carbide (SiC) and the Group III nitrides have larger bandgaps and thus can generate photons with greater energy that appear in the green, blue, violet and ultraviolet portions of the electromagnetic spectrum.
In some applications, an LED is more useful when its output is moderated or converted to a different color. As the availability of blue-emitting LEDs has greatly increased, the incorporation of yellow-emitting phosphors that down-convert the blue photons has likewise increased. The combination of the blue light emitted by the diode and the yellow light emitted by the phosphor can create white light. In turn, the availability of white light from solid-state sources provides the capability to incorporate them in a number of applications, particularly including illumination and as lighting (frequently backlighting) for color displays. In such devices (e.g., flat computer screens, personal digital assistants, and cell phones), the blue LED and yellow phosphor produce white light which is then distributed in some fashion to illuminate the color pixels. Such color pixels are often formed by a combination of liquid crystals, color filters and polarizers, and the entire unit including the backlighting is generally referred to as a liquid crystal display (“LCD”).
As the use of light emitting diodes has commercially increased and as the understanding of the basic characteristics of diodes used to produce white light has matured, the advances of interest in the technology tend to be those that increase the total amount of light that is produced by a given diode structure, all other factors being equal.
In this regard, the number of individual photons produced by a diode in any given amount of time depends upon the number of recombination events occurring in the diode, with the number of photons generally being less than the number of recombination events (i.e., not every event produces a photon). In turn, the number of recombination events depends upon the amount of current applied across the diode. Once again the number of recombination events will typically be less than the number of electrons injected across the junction. Thus, these electronic properties can reduce the external output of the diode.
Additionally, when photons are produced, they must also actually leave the diode and the lamp to be perceived by an observer. Although many photons will leave the lamp without difficulty, a number of well-understood effects prevent some fraction of the photons from leaving. These effects arise from the difference in refractive index of the various materials within the diode, and thus reduce the external output of an LED lamp (i.e., its efficiency). These include internal reflection of a photon until it attenuated and emitted or absorbed (i.e., Snell's Law and Fresnel Loss) rather than emitted. The difference in the index of refraction between the materials in the diode can also change the direction of an emitted photon (Snell's Law) towards an object that subsequently attenuates or absorbs it. The same results can occur for yellow photons that are emitted by the phosphor in a phosphor-containing LED lamp. In an LED lamp such “objects” can include the substrate, parts of the packaging, and the metal contact layers. Indeed, the same quantum mechanical characteristics that permit semiconductor materials to emit photons will also cause them to absorb photons. Thus, even the light emitting epitaxial layers in an LED can absorb emitted photons and reduce the overall external efficiency of the diode.
Many semiconductor devices, including many light emitting diodes, consist in basic form of a semiconductor substrate and epitaxial layers of semiconductor materials on the substrate. The epitaxial layers often (although not necessarily exclusively) form the active portions of the device. They are generally favored for this purpose because they are grown in a manner (frequently chemical vapor deposition) that increases both their chemical purity and produces a highly ordered crystal structure. Additionally, chemical vapor deposition provides an excellent technique for precisely doping an epitaxial layer. In turn, the appropriate purity, crystal structure and doping are typically desired or necessary for successful operation of the semiconductor device.
The chemical vapor deposition (CVD) and related techniques used to fabricate epitaxial layers are, however, generally more time-consuming than other crystal growth techniques such as sublimation or growth from a melt (sometimes referred to as bulk growth). As a result, these more rapid (comparatively) methods are often used to produce an appropriate crystal when the intended structure is other than an epitaxial layer.
Thus, by combining a bulk-growth substrate with epitaxial layers, an overall structure can be produced with a reasonable combination of crystal structure, compositional purity, doping, and efficient fabrication.
Nevertheless, for several crystal growth-related reasons, bulk (i.e., reasonably large size) single crystals of Group III nitrides are, for practical purposes, unavailable. Accordingly, Group IIII nitride LEDs are typically formed on other bulk substrate materials, most commonly sapphire (Al2O3) and silicon carbide (SiC). Sapphire is relatively inexpensive, widely available, and highly transparent. Alternatively, sapphire is a poor thermal conductor and therefore less suitable for certain high-power applications. Additionally, in some devices, electrically conductive substrates are preferred and sapphire is insulating rather than conductive. Sapphire also carries a lattice mismatch with (for example) gallium nitride of about 16%.
Silicon carbide has a better thermal conductivity than sapphire and a better lattice match with Group IIII nitrides; i.e., a mismatch of about 3.5% with gallium nitride and only about 1% with aluminum nitride. Silicon carbide can be conductively doped, but is also much more expensive than sapphire.
Although silicon carbide offers advantages for the growth of Group III nitride epitaxial layers, there are other reasons that encourage the use of other substrate materials in the final diodes. In order to use such other materials, the growth substrate (typically silicon carbide) must be partially or entirely removed and a carrier substrate must be added.
Depending upon the function and use of the final diode, the use of such carrier substrates offers several advantages. As one, the thickness required for a growth substrate during the diode fabrication steps may not be required in the final diode. By removing the growth substrate and replacing it with the carrier substrate, the overall thickness of the diode can be advantageously reduced. This is described in, for example, co-pending and commonly assigned application Ser. No. 10/951,042 filed Sep. 22, 2004 for “High Efficiency Group III Nitride-Silicon Carbide Light Emitting Diode.”
As another advantage, replacing the growth substrate with the carrier substrate often results in positioning the carrier substrate on the opposite side of the active layers from the growth substrate. For example, silicon carbide growth substrates are frequently n-type. Thus, the first epitaxial layer grown on the silicon carbide substrate is frequently an n-type Group III nitride layer. The p-type layer is then grown on top of the n-type layer.
The carrier substrate is then typically added to the p-type layer to form an intermediate structure having both substrates (growth and carrier). When the growth substrate is removed from the n-type layer, the carrier substrate remains attached to the p-type layer. The resulting structure has a carrier substrate, a p-type layer on the carrier substrate, and an n-type layer as the portion opposite the carrier substrate.
Although p-type layers are necessary for producing p-n junctions and junction characteristics, the conductivity of p-type Group III nitride materials is comparatively lower than that of n-type layers. As a result, it can be difficult to obtain a desired amount of current spreading in a p-type layer.
By using a carrier substrate, the epitaxial layers can be flipped and the p-type layer can be conductively mounted to the carrier substrate and the n-type layer can form the emitting face of the diode. In this orientation the higher conductivity of the n-type layer offers advantages in lateral current flow and thus light extraction.
As yet another advantage, and although the observation to date has been empirical, increased brightness has been observed from Group III nitride light emitting diodes in which the epitaxial layers are grown on silicon carbide and after which the silicon carbide substrate is removed and replaced with a carrier substrate.
Copending and commonly assigned application Ser. No. 11/338,918 filed Jan. 25, 2006 and now published as No. 20060131599 offers some additional explanations and considerations as to how the substrate can affect the overall device performance.
A carrier substrate can also provide a structure that is more amenable than a silicon carbide substrate to certain soldering techniques or other later fabrication steps.
In other cases, the working diode in context does not require the thermal or electronic or optical properties of silicon carbide. In such cases silicon carbide offers advantages for growth, but not for use. This differs, of course, from certain power devices in which the intrinsic semiconductor characteristics of silicon carbide are the relevant property. Growing Group III nitride epitaxial layers on silicon carbide and then removing the silicon carbide substrate can reduce the overall cost of the resulting diodes, because the removed silicon carbide substrate (which typically is used as a wafer and then is removed as a wafer) can be reused. Thus, although silicon carbide is comparatively more expensive than sapphire or other substrate materials, reusing it in this fashion moderates the cost of fabrication while providing the growth advantages of SiC for Group III nitride epilayers.
For at least these reasons, producing Group III nitride light emitting diodes on carrier substrates after growth substrates have been removed remains of significant interest and drives a desire for continuing improvements in the technology. Additionally, increasing the external quantum efficiency of light emitting diodes within the context of such carrier substrate structures remains a continuing goal.